Brush seals are used to minimize leakage through a gap between two components, wherein such leakage is from a higher pressure area to a lower pressure area. Such leakage is greater when the tips of the bristles of the brush seal contact a rough rotating surface of one of the components than when they contact a smooth rotating surface of the one component. A soft brush seal will not smooth out the rough surfaces of the one component and may itself be worn by such rough surfaces, either or both of which results in unacceptable seal leakage. An abrasive brush seal will smooth out the rough surfaces of the one component. However, Applicants have found that the abrasive bristles will not seal well if they do not wear evenly, which typically is the case.
Brush seals have been used, or their use proposed, in rotating machinery. Such use includes, but is not limited to, turbomachinery including steam turbines and gas turbines used for power generation and gas turbines used for aircraft and marine propulsion. It is noted that brush seals minimize the leakage of steam in steam turbines and minimize the leakage of compressed air or combustion gases in gas turbines.
A steam turbine has a steam path which typically includes, in serial-flow relationship, a steam inlet, a turbine, and a steam outlet. A gas turbine has a gas path which typically includes, in serial-flow relationship, an air intake (or inlet), a compressor, a combustor, a turbine, and a gas outlet (or exhaust nozzle). Gas or steam leakage, either out of the gas or steam path or into the gas or steam path, from an area of higher pressure to an area of lower pressure, is generally undesirable. For example, gas-path leakage in the turbine or compressor area of a gas turbine, between the rotating tips of the turbine or compressor rotor blades and the circumferentially surrounding turbine or compressor casing, will lower the efficiency of the gas turbine leading to increased fuel costs. Also, steam-path leakage in the turbine area of a steam turbine, between the rotating tips of the buckets (i.e., blades) and the circumferentially surrounding casing, will lower the efficiency of the steam turbine leading to increased fuel costs.
Conventional brush seal designs have been proposed for use in such applications. The brush seals have wire or ceramic bristles conventionally welded or otherwise affixed to a backing plate. To improve performance, such brush seals typically align their wire bristles to contact the rotating tips of the blades/buckets at an angle between generally forty-five and generally sixty degrees with respect to a radius line from the center of the rotor to the point of bristle contact. However, the rotating tips of the blades/buckets typically present a rough surface which has sealing problems as previously discussed.
What is needed is an improved brush seal which will maintain its sealing qualities even when used against rough rotating surfaces such as the shrouded free ends of gas turbine blades or steam turbine buckets.